1. Field of the Invention
The present invention relates generally to braided structures and more particularly to conformable braided structures that are characterized by axial sites and bias sites, where the axial sites have a greater diameter than the bias sites. The conformable braided structures are mathematically designed to meet the perimeter and area requirements of target gap area to be filled in a structure.
2. Background of the Invention
Resin transfer molding has been around for many decades, and its use has grown considerably in recent years. The process allows the economical manufacture of high quality composites. The term “composite” has been used principally to define a class of materials in which a matrix material, such as plastics (both thermosetting and thermoplastic), metals, or ceramics are reinforced by strengthening fibers in the form of a preform. Composites are advantageous since the final structure exhibits properties which are a combination of the properties of the constituent materials (i.e., the fiber reinforcement and matrix material).
In accordance with the process, a resin system is transferred at low viscosities and low pressures into a closed mold die containing a preform of dry fibers. The dry fibers, which may have the form of continuous strand mat, unidirectional, woven, or knitted preforms, are placed in a closed mold and resin is introduced into the mold under external pressure or vacuum. The resin cures under the action of its own exotherm, or heat can be applied to the mold to complete the curing process.
The resin transfer molding process can be used to produce low-cost composite parts that are complex in shape. These parts typically provide continuous fiber reinforcement, along with inside mold line and outside mold line controlled surfaces. It is the placement of the continuous fiber reinforcements in large structures that sets resin transfer molding apart from other liquid molding processes.
In the past, resin transfer molding was used for applications suitable to consumer product markets. However, in the last few years, through the development of high-strength resin systems and more advanced pumping systems, resin transfer molding has advanced to new levels. These recent developments have promoted resin transfer molding technology as a practical manufacturing option for high-strength composite designs, particularly in the aerospace industry.
In the aerospace industry, the most visible advantage to the resin transfer molding process lies in resin transfer molding's ability to combine multiple, detailed components into one configuration. For example, many traditional designs consist of many individual details that are combined as a subassembly. These subassemblies usually require labor-intensive shimming, bonding, mechanical fastening and sealing. Consequently, these subassemblies demonstrate high part-to-part variability due to tolerance build-up.
Resin transfer molding produces smooth surfaces. Being a product of the mold makes the surface quality of the part produced within the mold comparable to that of the tool's surface. Resin transfer molding also provides control of the fiber/resin ratio in the completed product. This advantage produces parts that are lightweight and high in strength.
However, when multiple detailed components having rounded edges are combined, the concavity of the edges causes gaps to be formed at the point where the components join together. Consider the geometries shown in FIGS. 1A and 1B, which are typical of the types of cross-sections that often need to be filled with “radius gap-filler” material during construction of a composite lay-up. One method to fill this volume with fiber is to lay-in individual ends of fiber. However, this method can be tedious and inefficient. An alternative method of filling this volume is to use a braided “gap-filler” component that holds multiple ends of the fiber together in a single piece. A drawback of this alternative is that gap filler is typically rigid and not easily conformable to varying cross-sections. Specialty braiders are available that can produce material to a specific geometry, but that geometry is then applicable only to a single application. Generic tubular braids are somewhat conformable to varying shapes, but geometric constraints of the braiding process make it difficult to obtain both the correct fiber volume and the correct perimeter for concave or complex shapes such as that of FIGS. 1A and 1B.
For example, consider again the geometry shown in FIG. 1A where the length of the sides of the concave gap or area 8 to be filled is indicated by 10 and 12 and the radius of the concave gap or area 8 to be filled is indicated by 14. In this example assume that the length of the sides, 10 and 12 of the concave gap 8 to be filled is 0.500 in. each. Further, assume that the radius of the concave gap 14 is approximately 0.500 in. In order to function as an acceptable gap filler that will be conformable to the shape of the gap, a braid must have roughly the same cross-sectional area (0.054 sq. in.) and perimeter (1.785 sq. in.) as the concave gap 8 in FIG. 1A. FIGS. 2A and 2B show the two extremes possible when trying to produce a circular or maypole braid to meet the area and perimeter criteria of FIG. 1A. In one case, as depicted in FIG. 2A where the braid radius 16 is 0.284 in., the perimeter is fixed at 1.785 in. and the area of the solid braid (0.253 sq. in.) is too high. In the other case, as depicted in FIG. 2B where the braid radius 16 is 0.131 in., the area is fixed at 0.054 sq. in. and the perimeter of the solid braid (0.283 in.) is too low.
In some cases it is possible to avoid the dilemma described above with conventional maypole braids via the use of mandrels. In fact, braids have the property of being conformable to mandrels of various cross-sections. However, this capability is limited in that the mandrel must have no concave geometry. Thus, one typically must attempt to braid around a convex geometry of the target perimeter, then deform this perimeter to the desired concave shape after braiding. However, the area bounded by the initial convex braid prior to deformation will always be higher than the target area of the concave geometry. This is shown in FIG. 3 by the circular cross-section of a braided sheath about an undersized core 18 with an idealized perimeter but also having a high internal void area 24. As opposed to the solid braid in FIG. 2A in which the area of fiber is too large, the braid in FIG. 3 shows a solid core of fiber 22 sized appropriately to meet the area requirement. Any attempt to fill the void area 24 between the solid core of fiber 22 and the braided sheath 20 with unidirectional fibers would be futile, as the core would simply “fall out” of the sheath.
U.S. Pat. No. 6,231,941 discloses a radius or gap filler to fill concave areas as depicted in FIGS. 1A and 1B. As disclosed, a braided sleeve surrounds a number of unidirectional tows (untwisted filaments). The core of the unidirectional tows can be of uniform cross section, or can be varied in cross-section along its length so as to fit a particular gap. The radius filler is formed on a mandrel that includes a contoured surface that is substantially the same shape as depicted in FIG. 1A or FIG. 1B. The braided sleeve is braided around the unidirectional tows and is then soaked with a tackifier. The braided sleeve with the unidirectional tows therein is then placed on the mandrel surface and is vacuum bagged under a bladder. The bagged radius filler is then placed in an autoclave and heat is applied while vacuum is applied to the bladder. The bagged radius filler is heated until the tackifier on the braided sleeve is procured or semi-hardened. Since the tackifier is only semi-hardened, it acts as a binding agent to maintain consolidation and configuration of the braided sleeve until the final transfer molding of the component to be filled is performed. However, this process requires specially designed mandrels to construct the specific gap filler required which is a time consuming, laborious and expensive process.
Accordingly, a need exists for a braided gap filler that can be designed so that it is conformable to gaps with varying cross sections, that can be constructed using conventional braiding techniques.